The present invention relates to protein hydrogels in general and protein-polysaccharide hybrid hydrogels in particular. The hydrogels disclosed herein are capable of absorbing large amounts of water or other liquids per unit mass.
Beginning in the early 1970""s, and continuing to the present day, there has been a growing awareness that the continued widespread use of non-biodegradable, petroleum-based polymeric materials may pose serious environmental concerns. These concerns are heightened by production statistics showing the enormous and still-growing volume of non-biodegradable plastics produced annually, the vast majority of which are ultimately interred in landfills. This raises concerns not only as to the amount of space available for solid waste disposal (which is disappearing at an increasingly rapid pace), but also raises equally serious concerns that the leaching of toxic monomers and oligomers from landfilled plastics will contaminate ground water, thereby causing health problems in humans and animals.
In addition to concerns regarding human health and the environment, the world-wide depletion of petroleum reserves, in combination with wildly fluctuating petroleum prices due to political and economic conflicts, indicates that less dependence on petroleum-derived products might be prudent. Therefore, the development of alternative, and renewable, resources for industrial products is needed.
Because of the factual and/or perceived economic, environmental, and public health concerns accompanying non-biodegradable, petroleum-based products, a non-petroleum-based, environmentally safe, biodegradable, and renewable source for industrial products is needed. As evidenced by the following references, several types of useful products have been fabricated from renewable sources of starting materials.
For instance, Mann, U.S. Pat. No. 2,729,628, describes a process for increasing the intrinsic viscosity of a long chain polypeptide, particularly natural proteins such as peanut protein, soybean protein, casein, egg albumin, and blood albumin by acylating the protein with terephthalyl dichloride. Here, the protein is reacted with the terephthalyl dichloride using the Schotten-Baumann method at a temperature of from about 0xc2x0 C. to 30xc2x0 C.
Young et al., U.S. Pat. No. 2,923,691, describe the polymerization of animal proteins to improve their characteristics for use as animal glue. Young et al. introduce aldehydes to an animal glue protein so as to modify the viscosity and jelly characteristics of the glue product without solidifying or insolubilizing the protein. Here, Young et al. are interested in increasing the viscosity and jelly strength of last run animal glues, which tend to be of inferior quality. The process described by Young et al. includes two steps: first, a cyanic acid salt is reacted with the protein material; second, an aldehyde, such as formaldehyde or glucose, is added to the protein material.
Two patents to Miller (U.S. Pat. Nos. 3,685,998 and 3,720,765), and assigned to the Monsanto Company, describe improved protein feed materials for ruminants. In the Miller patents, protein feeds are rendered resistant to digestive breakdown in the rumen, but not in the abomasum and intestines, by treating protein-containing feed material with a polymerized unsaturated carboxylic acid or anhydride. For instance, the proteinaceous feedstuff is treated with a polyanhydride such as poly(maleic anhydride). This renders the protein feedstuff substantially indigestible in the fluid medium of the rumen, yet still digestible in the acidic media of the abomasum and the intestines. In this manner the proteins of the feedstuff are spared breakdown in the rumen and are available for absorption in the subsequent digestive organs.
Three patent references to Battista (U.S. Pat. Nos. 4,264,493, 4,349,470, and 4,416,814) describe the formation of protein hydrogel structures formed from natural proteins having molecular weights not exceeding 100,000 by dissolving the protein in an aqueous acidic solution, cross-linking the protein, and air drying the solution to a moisture content not exceeding 10 percent. The Battista patents are largely drawn to the formation of clear products such as soft contact lenses, ophthalmological films, and the like.
Although Battista refers to the compositions described therein as hydrogels, that term is defined within the Battista references as meaning xe2x80x9ca cross-linked protein polymer of natural origin having an average molecular weight of about 100,000 or less, capable of being swollen by water over a wide range of water contents ranging from as low as 30 percent to 1,000 percent and higher while possessing useful theological control properties for a specific end product uses.xe2x80x9d The hydrogels described by Battista are not designed to be superabsorbent. Rather, they are designed to be optically clear and to have sufficient mechanical integrity to function as soft contact lenses.
The protein hydrogel structures described in the Battista patents are made from natural protein raw materials that form clear solutions in water. The protein raw material is first dissolved in an acidic aqueous solution of from pH 3.5 to about pH 5.5. A cross-linking agent is then added to the acidic protein solution. Battista""s preferred cross-linking agent is Formalin (37% formaldehyde); however, Battista describes other suitable cross-linking agents which may be used, including glutaraldehyde. It must be noted, however, that the Battista patents do not describe acyl-modification of the protein starting material. Nor do the Battista patents describe a superabsorbent protein hydrogel. The protein hydrogels described in the Battista references are designed to have increased wet strength capabilities, thereby enabling their use in soft contact lenses.
Many disadvantages which accompany synthetic hydrogels (such as non-biodegradability) can be overcome by using hydrogels derived from natural polymer sources. In addition to chemically cross-linked protein hydrogels, such as those described by Battista, many proteins can be thermally induced to form gels. The most critical requirements for any type of biopolymer hydrogel are that the gel should have the capacity to absorb a large amount of water relative to its mass upon rehydration, and that the gel material itself should resist dissolution.
However, conventional thermally-induced protein hydrogels do not swell to their original gel volume after they have been dehydrated. This decreased swelling capacity is related to increased hydrogen bonding, as well as electrostatic and hydrophobic interactions which occur in the dehydrated protein. The loss of swelling of thermally-induced protein hydrogels limits their range of industrial applicability.
Perhaps the most desirable of renewable production materials is agricultural biomass. This is due, in large part, to the tremendous amount and variety of agricultural products which are produced in the United States. For instance, biomass (mainly maize) is currently used to produce ethanol for fuel. Fibrous biomass is widely used in the paper and forest products industry. Starch-derived products are also widely utilized in various industrial applications, such as the packing industry, in addition to their use in the food industry.
However, among biopolymers, proteins are perhaps the most under-utilized and under rated in terms of their industrial applications. They are primarily regarded solely as functional and nutritional ingredients in foodstuffs. Their enormous potential as structural elements in non-food industrial applications is largely unrecognized and unrealized. This is unfortunate because proteins offer several distinct advantages over more conventional types of biomass.
For example, unlike polyol-based natural polymers, such as cellulose and other carbohydrates, proteins contain several reactive side groups, including amino, hydroxyl, sulfhydryl, phenolic, and carboxyl moieties. These reactive groups can be used as sites of chemical modification and cross-linking to produce novel polymeric structures.
As a generic class of polymers, hydrogels of all types find high-volume uses in industrial applications, consumer products, and environmental applications. Such applications include diapers, catamenial devices, and industrial absorbents. As used herein, the unqualified term xe2x80x9chydrogelxe2x80x9d refers to any naturally-occurring or synthetic material which exhibits the ability to swell in water or some other liquid and to retain a significant fraction of liquid within its structure, but which will not dissolve in the liquid.
Several synthetic hydrogel materials are currently in use. These include such synthetic hydrogels as poly(hydroxyalkyl methacrylates), polyacrylate, poly(acrylamide), poly(methacrylamide) and derivatives thereof, poly(N-vinyl-2-pyrolidone), and poly(vinylalcohol). While these synthetic hydrogel polymers exhibit several interesting properties, their use in industrial, consumer, and environmental applications is less than desirable because of the toxicity of residual monomers and oligomers which are normally present in these gels. Moreover, the poor biodegradability of these synthetic hydrogels also poses the long-term environmental concerns discussed previously.
Most proteins are known to form thermally-induced gels under appropriate conditions. The most critical requirement for any biopolymer to behave as a hydrogel is that it should have the capacity to absorb a large amount of water upon rehydration. As noted earlier, however, in the case of heat-induced protein gels, they do not even swell to their original gel volume once they are dried.
The inability to re-swell is related to increased protein-protein interactions that arise as a result of dehydration. These interactions involve hydrogen bonding, and electrostatic, hydrophobic, and van der Waals forces. It follows then that if these attractive protein-protein interactions are sufficiently weakened and the structure of the polypeptide chain is turned into a more flexible random coil, it should be possible to improve the swelling and water absorbing properties of protein gels. This can, in principle, be achieved through appropriate chemical modification of the side-chain residues. For example, introducing a large number of negative charges, or converting positive charges to negative charges or vice versa, or attaching polyols at critical locations in the polypeptide chain, should increase not only the potential sites for water binding, but also repulsive interactions between protein segments and thus favor protein-solvent instead of protein-protein interactions. The higher the number of charged groups introduced, the higher would be the repulsive interaction and thus the water absorbing properties of the gel. Using this rationale, it is indeed possible to develop protein-based hydrogels having functional properties not found in conventional thermally-induced protein hydrogels.
One of the classes of chemical reagents that has been used for this purpose is ethylenediaminetetraacetic dianhydride (EDTAD): 
Ethylenediaminetetraacetic Dianhydride (EDTAD)
Reacting EDTAD with the xcex5-amino group of lysine residues in proteins results in formation of an isopeptide derivative as shown below: 
In the above reaction, for each lysyl residue modified (i.e., for each positive charge removed), three negative charges are added to the protein. Thus, a highly polyanionic protein polymer can be synthesized using the above reaction. Cross-linking of the polyanionic polymer by using bifunctional reagents, such as glutaraldehyde, yields an insoluble hydrogel with very high water-absorbing properties.
Recently, it has been shown that chemical modification of soy protein with EDTAD converted the protein into a poly-anionic polymer. When this protein polymer was cross-linked using a bifunctional reagent, the protein was converted into a superabsorbent protein hydrogel. These soy protein hydrogels can absorb 100-350 g water per g of gel, depending on the number of carboxyl groups (i.e., EDTA) attached to the protein. These gels are also capable of absorbing about 13-15 of 0.9% saline solution per g of dry gel within 30 min. They are capable of chelating divalent cations, such as lead, mercury, and cadmium ions. They are completely hydrolyzed by soil microbes within 3-4 weeks, suggesting that they can be composted. See U.S. Pat. No. 5,847,089, to Damodaran and Hwang, and U.S. Pat. No. 6,310,105, to Damodaran.
However, there still exists a need for improved, biodegradable, superabsorbent, biomass-derived hydrogels which exhibit reversible swelling, and which have improved functional qualities, such as absorption under load and centrifugal retention capacity.
In light of the above-noted shortcoming in the prior art, the present invention is directed to a hybrid protein-polysaccharide hydrogel. In a first embodiment of the invention, the hydrogel comprises two interpenetrating matrices: a first matrix comprising an acylated, cross-linked protein matrix; and a second matrix comprising an anionic polysaccharide matrix. The two matrices are interpenetrating, thereby yielding a homogeneous hydrogel that has superior saline absorption and retention characteristics as compared to hydrogels fabricated solely from protein matrices. In this first embodiment, the protein matrix and the polysaccharide matrix are not cross-linked to one another.
A second embodiment of the invention is drawn to the hydrogel as described in the immediately preceding paragraph, with the hydrogel further comprising bridging moieties that covalently link the acylated, cross-linked protein matrix to the anionic polysaccharide matrix. The bridging moieties may be uniformly dispersed throughout the bulk of the hydrogel gel, or the bridging moieties may be introduced after formation of the non-cross-linked hydrogel so as to form bridging moieties only on the surface of an otherwise non-cross-linked gel particle. In this fashion, each individual particle comprises a core of non-cross-linked hydrogel according to the present invention, enveloped within an outer shell of cross-linked hydrogel according to the present invention.
In either the first or second embodiment of the invention, the protein matrix can be derived from any protein source, without limitation, including biomass, protein isolates derived from biomass, soy bean protein isolate, protein derived from fish protein derived from other animal sources, such as from blood, etc.
The anionic polysaccharide can also be derived from any source, without limitation. By way of example, and not limitation, the anionic polysaccharide can be selected from the group consisting of alginates, carrageenans, carboxylated starches, carboxy-(C1-C6-alkyl) cellulose, gellans, hyaluronic acid, pectins, and xanthans. Carboxymethyl cellulose is preferred.
In the preferred method of fabrication, the acylated, cross-linked protein matrix is produced by adding carboxyl moieties to lysyl residues of a protein matrix, to yield an acylated protein matrix, and then crosslinking the acylated protein matrix with a bifunctional cross-linking reagent, to yield the ayclated, cross-linked protein matrix. In the preferred route, carboxyl moieties are added to the lysyl residues of the protein matrix by treating the protein matrix with ethylenediaminetetraacetic acid dianhydride (EDTAD). It is then preferred that the acylated protein matrix is cross-linked using a bifunctional cross-linking reagent selected from the group consisting of
OCHxe2x80x94(CH2)xxe2x80x94CHO
wherein X is an integer of from 2 to 8. Glutaraldehyde is the preferred cross-linking reagent.
To create the bridging moieties in the cross-linked hydrogel, the interpenetrated acylated, cross-linked protein matrix and the anionic polysaccharide matrix are treated with a bifunctional bridging reagent, preferably a diglycidyl ether or ethylene carbonate. This induces the formation of ester bonds linking the carboxyl groups of the protein matrix with the carboxyl groups of the anionic polysaccharide matrix. The preferred bifunctional bridging reagent is a C2-C16-alkylene diglycidyl ether and ethylene carbonate, with ethylene glycol diglycidyl ether being the most preferred.
In view of the above discussion, it is a principal aim of the present invention to provide a protein- and polysaccharide-based hydrogel which is superabsorbent, reversibly swellable, biodegradable, and which possesses superior physical characteristics such as absorption under load and centrifugal retention capacity. The hydrogel of the present invention absorbs and retains saline far better than conventional, protein-based hydrogels, thus making the subject hydrogel ideal for use in diapers. The subject hydrogel is also, unlike hydrogels based on synthetic polymers, biodegradeable.
A further aim of the invention is to provide a protein- and polysaccharide-based hydrogel which can be formed from a wide range of protein and polysaccharide starting materials, and which can be used as a substitute for wholly synthetic hydrogels.
The protein from which the subject hydrogel is fabricated can be from any plant or animal source, without limitation. A preferred protein source, its preference derived in large part from its abundance and low cost, is soy-derived protein. Likewise, the polysaccharide from which hydrogel is fabricated can be from any source, without limitation.
In operation, the protein hydrogel can be used wherever high absorption and retention of liquid is desired. Potential end uses for the hydrogels of the present invention include diapers, tampons and menstrual pads, industrial absorbents, spill dams and sealers, drilling muds, ground and waste water reclamation applications, heavy metal sequestration, and the like. These and many other utilities for superabsorbent hydrogels described herein are well within the purview of one of skill in the field of hydrogels.